As a device for measuring an intake airflow rate in an internal combustion engine, a system, in which a heat resistor with temperature-dependence and a resistor (temperature-compensating resistor) for temperature-compensation of the heat resistor are disposed in an intake air passage (air passage) to measure the intake airflow rate, is well known.
With this type of an airflow meter, even if heat of the heat resistor is released into airflow, a heating current flowing into the heat resistor is controlled so that a temperature difference between the heat resistor and the temperature-compensating resistor becomes constant, and a change in the current is directly or indirectly sensed, thereby measuring an airflow rate.
In the case where such a thermal airflow meter as described is installed in an internal combustion engine of an automobile, and so forth, heat of the internal combustion engine is transferred from the wall of the intake air passage to the airflow meter due to rise in temperature of the engine. Further, there are times when air temperature undergoes a change while the automobile is running. Further heat generation occurs to part of the airflow meter itself, such as a drive circuit for driving the airflow meter. Such a change in ambient temperature and self-heating cause an error in a sensed flow rate of the airflow meter even if the temperature-compensating resistor is provided.
Conventional thermal airflow meters designed so as to reduce a flow rate sensing error due to such a change in temperature, are disclosed in Japanese Patent Laid-Open No. S61(1986)-239119, and Japanese Patent Laid-Open No. H10(1998)-197309, respectively.
Japanese Patent Laid-Open No. S61(1986)-239119 is described as follows. It is empirically known that if temperature of an air passage body through which air to be measured flows differs from a sensed air temperature, this will cause a airflow rate sensing error. And, the sensing error is corrected in the following manner. That is, a resistor for measuring an air temperature is disposed in an air passage, and a wall-face of the air passage is provided with a temperature-sensing resistor for sensing temperature at the wall face. Thereby an airflow rate signal is corrected on the basis of a difference between the temperature at the wall face of the air passage and the air temperature.
Further, in Japanese Patent Laid-Open No. H10(1998)-197309, it has been disclosed that a sensor for sensing an air temperature and a sensor for sensing a substrate temperature are provided on a substrate having a flow-rate measuring element (a heat resistor and a temperature-compensating resistor). The substrate of the flow-rate measuring element is structured in such way as to be held by a holder attached to the wall of an air passage. The substrate-temperature sensor is provided at one end side of the substrate adjacent to the holder. Meanwhile, the air-temperature sensor is disposed at the other end side of the substrate (opposite to the holder). The substrate-temperature sensor senses rise in the substrate temperature due to heat-transfer from an engine and so forth. The heat of the engine is transferred from the wall of the intake air passage to the substrate of the flow-rate measuring element. On the basis of information on the substrate temperature, and information on the air temperature, a flow rate sensing error is corrected.
In the case of conventional arts described in the foregoing, according to a system in which the wall-face of the air passage is provided with the temperature-sensing, as disclosed in Japanese Patent Laid-Open No. S61(1986)-239119, the temperature at the wall face is sensed by embedding the temperature-detection resistor in the wall of the air passage. With such a configuration as described, however, the temperature-sensing resistor is embedded in resin, so that a molding process of the airflow meter becomes complex. Furthermore, in order to draw out a terminal of the temperature-sensing resistor to the outside of the wall of the air passage, it has been necessary to additionally provide a body of the airflow meter with an external connection terminal for temperature output besides a terminal for an airflow rate signal, and a power input/output terminal.
Meanwhile, in Japanese Patent Laid-Open No. H10(1998)-197309, the substrate-temperature sensor and the air-temperature sensor are provided on the substrate with the flow-rate measuring element (the heat resistor and the temperature-compensating resistor), and obtaining an airflow rate according to a temperature difference between both the sensors. With this system, in the case where a semiconductor substrate made of silicon and so forth is used for the substrate of the flow-rate measuring element, because a silicon substrate has a high thermal conductivity among substrate materials, there occurs little difference between the one end of the substrate (adjacent to the holder) and the other end of the substrate (opposite to the holder). Accordingly, the information on the substrate temperature, and the information on the air temperature come to indicate nearly an identical temperature all the time. So, it is impossible to satisfactorily sense heat transferred to the substrate from outside such as the engine and so forth, and the airflow meter has a matter to be improved in respect of correction accuracy.
Furthermore, because heat from outside such as the holder and so forth is transferred to the flow-rate measuring element in whole, the air temperature information obtained from the intake air-temperature sensor had a tendency of indicating temperature higher than an actual air temperature.